Definitions and Concepts for CAIE Biology IGCSE
Topic 1: Characteristics and Classification of
Living Organisms
Definitions in ​bold ​are for supplement only
Amino acids​ - Small molecules from which proteins are synthesised.
Amphibia ​- A class of vertebrates - including frogs, toads and newts - that are
poikilothermic. They have four limbs, of which hind feet are often webbed. Amphibia have
lungs and moist skin for breathing. They reproduce sexually via external fertilisation.
Animal (kingdom)​ - A biological kingdom consisting of multicellular organisms that do not
have a cell wall or chloroplasts. They ingest and digest food for energy.
Arachnids ​- A class of arthropods including spiders, scorpions, ticks and mites. Arachnids
have a segmented body (divided into the cephalothorax and abdomen), four pairs of jointed
legs and several pairs of simple eyes.
Arthropods ​- A phylum consisting of invertebrate animals with jointed limbs, segmented
bodies and a hard external skeleton (cuticle). Arthropods include crustaceans, myriapods,
arachnids and insects.
Binomial system ​- A universal system for naming organisms using their genus and species.
Birds ​- A class of vertebrates that are homoiothermic. Birds have four limbs (the forelimbs
form wings), and beaked jaws. They have an outer covering of feathers over most of the
body and scales on the legs and toes. Fertilisation occurs internally, with the female laying
hard-shelled eggs.
Cell membrane ​- The partially permeable barrier that surrounds a cell.
Classification​ - The organisation of organisms into groups based on their characteristics
and structure. ​Classification systems may also reflect evolutionary relationships
between organisms.
Crustacea ​- A class of arthropods including crabs, lobsters, prawns, shrimps, the water
louse and woodlice. They have compound eyes and two pairs of antennae.
Cytoplasm ​- The jelly-like component of cells that contains sub-cellular structures, dissolved
nutrients and salts.
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Dichotomous key ​- A tool consisting of a series of choices (i.e. features) used to identify an
unknown organism.
Dicotyledons ​- Plants that produce seeds that contain two cotyledons (embryonic
leaves), resulting in two primary leaves.
DNA ​- A double-stranded polymer, wound to form a double helix, that carries the
genetic code.
DNA base sequence​ ​- The sequence of bases in a molecule of DNA which can be used
as an accurate method of classification. The more similar the base sequences, the
more closely related the organisms.
Enzymes ​- Biological catalysts that increase the rate of reaction in living organisms.
Excretion​ - The process by which toxic substances and excess materials are removed from
the body.
Excretion​ - The process by which metabolic waste, toxic substances and excess
materials are removed from the body.
Ferns​ - A group of vascular land plants that have neither seeds nor flowers and
reproduce via spores from numerous sporangia. They have simple, fleshy
underground stems known as rhizomes.
Fish ​- A class of vertebrates that are poikilothermic. They have a smooth-streamlined shape
with fins for movement and are covered in overlapping scales. Fish have gills for breathing
and reproduce sexually, with fertilisation normally occurring externally.
Fungi ​- A biological kingdom consisting of organisms made up of hyphae (long,
branching filamentous structures). They reproduce asexually by producing spores.
Growth​ - A permanent increase in size.
Growth​ - An increase in size or dry mass that is permanent. This may be due to an
increase in the number or size of cells​.
Homoiothermic​ - Describes an organism that is warm-blooded and whose body
temperature is internally regulated.
Insects ​- A diverse class of arthropods including butterflies, bees, beetles, mosquitoes,
houseflies, greenfly and earwigs. They have segmented bodies (consisting of a head, thorax
and abdomen), three pairs of jointed legs, one pair of antennae, compound eyes and
normally two pairs of wings.
Kingdom​ - The largest group of organisms identified by biologists. ​There are five
kingdoms: Animal, Plant, Fungus, Prokaryote and Protoctist.
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Mammals ​- A class of vertebrates that are homoiothermic. They have four limbs and are
characterised by the presence of hair or fur, and mammary glands. Mammals produce live
young.
Metabolism​ - All of the chemical reactions that take place inside an organism.
Monocotyledons ​- Plants that produce seeds that contain one cotyledon (embryonic
leaf), resulting in one primary leaf.
Movement​ - A change in position due to the action of an organism.
Movement ​- A change in position due to the action of an organism or part of an
organism.
Myriapods ​- A class of arthropods consisting of millipedes and centipedes. They have a
segmented body, ten or more pairs of jointed legs, one pair of antennae and simple eyes.
Nutrition​ - The process by which living organisms take in materials necessary for growth,
development and energy. ​Animals require organic compounds, ions and water. Plants
require light, carbon dioxide, ions and water.
Plant ​(kingdom)​ ​- A biological kingdom consisting of multicellular organisms that have a
cellulose cell wall and can photosynthesise.
Poikilothermic​ - Describes an organism that is cold-blooded and whose body temperature
varies with the temperature of the external environment.
Prokaryote ​- A biological kingdom consisting of unicellular bacteria which lack a
nucleus and membrane-bound organelles.
Protein ​- A large molecule that is synthesised from amino acids.
Protoctista ​- A biological kingdom consisting of unicellular eukaryotes.
Reproduction​ - The production of more of the same type of organism.
Reptiles ​- A class of vertebrates that are poikilothermic. They have four limbs and dry, scaly
skin. Reptiles reproduce sexually, with fertilisation occurring internally.
Respiration​ - Chemical reactions that take place inside cells and release energy from the
breakdown of nutrient molecules. ​This energy is used in metabolism.
Ribosomes ​- Sub-cellular structures that are the site for protein synthesis.
Sensitivity ​- The ability to sense and respond to environmental change.
Sensitivity​ - The ability to sense and respond to internal and external stimuli in the
environment.
Species​ - A group of similar organisms that can interbreed to produce fertile offspring.
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Vertebrates ​- A group of animals which have a vertebral column (otherwise known as the
‘spinal column’ or ‘spine’). Vertebrates include fish, amphibia, reptiles, birds and mammals.
Virus ​- A non-living microorganism that consists of genetic material surrounded by a
protein coat.
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